This invention relates to a polyimide precursor resin composition and semiconductor devices such as hybrid IC, monolithic IC, LSI, etc., using said composition.
Polyimide resins are increasingly used recently for the interlaminar insulating films or surface protective films (which are called passivation films in some cases) in semiconductor devices. This owes to the moisture-resistant protective properties of polyimide resins for wiring materials such as Al (aluminum) and their ease of formation into thin films.
A typical example of the use of polyimide resin for a surface protective film of a semiconductor device is described below with reference to the accompanying drawings.
Referring to FIG. 2 of the accompanying drawings, there is illustrated a semiconductor chip comprising a single-crystal silicon (Si) semiconductor substrate 1 incorporated with semiconductor elements such as diodes, transistors, etc., a 0.4-0.6 .mu.m thick silicon oxide (SiO.sub.2) film or phosphorous glass film 2, 3 and 4 covering the main surface of said semiconductor substrate 1 and partly opened by photoetching, and an Al wiring layer constituting a first conductor layer (sublayer) 5, 6 and 7 in resistance contact with the emitter, base and collector regions respectively and overlying said silicon oxide or phosphorous glass film 2, 3 and 4. The Al wiring layer is formed by vacuum evaporation or sputtering and the unnecessary portion is removed by photoetching to form a pattern. This Al wiring layer is, for instance, 1.5 .mu.m thick and 7 .mu.m wide.
Then, usually a solution of an aluminum chelate compound is coated by a spinner or other suitable means on the silicon oxide or phosphorous glass film and Al wiring layer of FIG. 2 and subjected to a heat treatment at about 250.degree. C. for 30 minutes to form an approximately 100-200 .ANG. thick aluminum oxide film 8 as shown in FIG. 3. Then an ordinary polyimide resin precursor polyamide acid solution is further coated by a spinner or other means to form a polyimide resin film 9 to complete a semiconductor device.
Generally, polyimide resin film has poor adhesion to silicon oxide film, Al wiring layer, etc., so that an aluminum chelate treatment is applied to improve the adhesive force. This adhesive force has a close relation to the moisture resistance reliability of the semiconductor device, and in fact moisture resistance of the semiconductor device is greatly improved by increasing the adhesive force.
Generally, the following methods are known for improving adhesiveness of polyimide resin: (1) the substrate surface is treated with a solution containing a primer such as aluminum chelate, silane coupling agent, etc., before coating with a poly(amic acid) such as mentioned below; (2) a silane coupling agent is added to the poly(amic acid) solution immediately before use; (3) a diaminosiloxane is copolymerized to introduce siloxane linkages to the poly(amic acid).
The method (1), however, has a problem that the wafer process is complicated. In the case of the method (2), in order to afford a sufficient adhesiveness to the polyimide resin film, a silane coupling agent must be added in an amount of 5% or more by weight based on the poly(amic acid) solution, but the addition of such a large amount of silane coupling agent greatly reduces the viscosity stability of the poly(amic acid) solution and invites excess bodying-up of the solution, which may even result in gelation of the solution, so that it is impossible to keep in storage the poly(amic acid) solution in a state added with a silane coupling agent. In the method (2), therefore, in case the produced semiconductor device is applied to electronic instruments or apparatuses in which even slight contamination with alien matter could be a cause of serious trouble, the silane coupling agent must be added just before use and succeeding filtration for eliminating alien matter is indispensable. This greatly handicaps the process. According to the method (3), although the adhesive force is sufficiently improved at room temperature, this adhesive force drops sharply and the moisture resistance reliability of the semiconductor device is also lowered when the product is subjected to a pressure cook test at 121.degree. C. under 2.1 atm.
Semiconductor devices are usually sealed with a thermosetting resin in practical use. The primary reason for use of resin seal is low material cost. Another reason is the ease of rationalization of production leading to a cost reduction. However, examination by the present inventors of moisture resistance of said resin sealed semiconductor devices disclosed that Al wiring of the conductor layer suffers from corrosion when the semiconductor device is left under a harsh condition for a long time as the resin itself has the nature to allow passage of moisture from the outside. It was thus found that said resin sealed semiconductor devices have a problem in reliability in their use in the fields where a high degree of reliability is required, especially in the industrial uses.
In order to enhance reliability of semiconductor devices, some ideas have been introduced such as use of Al containing a few percent of Si (silicon) for the conductor layer. Use of such Si-containing Al, however, involves a problem of elevated material cost.
Said defect of resin seal, when considered here with reference to a semiconductor device having a double-layer wiring structure, may be attributed to the following reason: in the resin sealed semiconductor device, the moisture which has penetrated into the device from the outside through the resin sealant is reacted with Al wiring of the conductor layer for some reason or other, resulting in inviting corrosion.
Polyimide resin film is almost free of crack and therefore has no possibility of moisture penetration through cracks, but on the other hand, such polyimide resin film, being an organic matter, has an innate tendency to absorb water into the inside and this is considered responsible for possible corrosion of Al wiring. As a result of investigations of semiconductor devices using Al for conductor layer and polyimide resin for insulating layer, the present inventors found that in the case of double-layer wiring, Al of the conductor layer forming the upper-layer wiring in contact with resin layer is far more liable to corrosion than Al of the conductor layer which is not in contact with resin layer.
This fact dictates that the corrosion of Al conductor layer is not only related to hygroscopicity of polyimide resin layer but is also associated with the adhesive strength at the interface between the conductor layers and the sandwiched polyimide resin films or between the individual polyimide resin films, and it is thus considered that if said adhesive strength is low, the absorbed moisture exists at the interfaces to cause corrosion of Al conductor layers. Especially in the case of said double-layer wiring, since the adhesive strength at the interface between the polyimide resin forming the intervening insulating layer and the conductor layer or between the polyimide resin films is low, the conductor layer in contact with the polyimide resin layer is more likely to be corroded than the conductor layer not in contact with said polyimide resin film.